The 41-year-old, who helped form I Like Clean Air, started measuring the pollution level at intersections where the staggered traffic lights meant she and her children often spent long periods waiting in the middle of busy roads.

"There are trucks and buses and concrete mixers and lorries and most of them run on diesel and we're stuck in the middle of the road," Mrs Ali-Webber said.

"We do know from the studies that children's lungs are being stunted.

"And the worst thing is that stunting stays with them into their adult lives so the respiratory problems that occur will stay with them."

In 2015, a study by researchers at London's Kings College found about 9,500 people died each year due to long-term exposure to the pollution.

Paul Drayson, a former Labour Party science minister and the CEO of Drayson Technologies, has come up with a product he hopes will provide the most complete digital map of the city's air.

"If you have a device that tells you where the air is good and where it is not so good you can avoid those pollution hot spots and you can look after your health," Lord Drayson said.

"Whilst people are hopefully getting on with removing the internal combustion engine from our cities and improving the air, people can actually make a difference for themselves."

While the tag does not measure nitrogen oxides, it does track carbon monoxide (CO).

"More CO indicates higher levels of more harmful substances, such as nitrogen oxides and tiny soot particles that lodge in the lungs," Lord Drayson said.

The app developers are working with bicycle couriers who zip around the city with the tags in their backpacks adding to the pollution statistics being fed into the digital map.

London pollution 'less obvious' than smog in India, China

Simon Birkett is another passionate anti-pollution agitator.

The former banker retired early to campaign full time against poor air quality.

"Particle levels are much higher in China, India or even Eastern Europe than they are here," he said.

"But for gases like NO2, a toxic gas, scientists say London has the highest level in the world, so that is what we are really worried about."

He said people were still coming to terms with the dangers London's bad air poses, especially because clear skies make them think the problem is not as serious as it is in Beijing or Delhi.

"I think when you spend time in those cities it's obvious they've been living with it for years. You don't have to convince them that they have a serious problem," he says.

"Here in London it's less obvious but … it is having a significant effect on our health and we don't want it to get as bad as it is on those cities.

"Nitrogen dioxide will affect everyone to a greater or lesser extent but it certainly exacerbates respiratory problems.

"It's also been associated with heart attacks and strokes, so it is a serious problem."

Diesel cars and other heavy vehicles are blamed for much of London's air problems.

Mr Khan has said he will more than double funding to clear up the capital's dirty air, improving the quality of buses and encouraging taxi drivers to switch from the older black cabs to new ones with zero emissions.